The basic idea of relativity in physics is that the values of dynamic
variables can only be given relative to a frame of reference. Motion
is to be seen as motion with respect to a frame of reference,
which may be itself in motion when observed from another frame
of reference. Physical laws are to be invariant with respect to the
symmetry of transformations among the choice of admissible class
of frames of references. Naively, a frame of reference is an abstraction
of an idealized physical system. In practice, we can only have actual,
less than ideal, physical systems to be used. That does not raise any
new theoretical concern so long as the latter can be considered
classical. Some implications from the quantum nature of the physical
system to be used as the frame of reference has been discussed
back in 1967 [1]}. Before the turn of the century, however,
the only more notable papers addressing the topic are, apparently,
Refs.[2]}, [3]}. The last one was inspired from consideration of
candidate quantized theory of gravity. The general notion of
so-called relational formulation of physics has been highlighted.
A simple way to put it is that there is no absolute frame of reference,
which is really the relativity principle. All physical quantities and
states are abstract notions the explicit description of which is
reference frame dependent. With respect to quantum physics,
described in the usual language, not only the expectation value
of an observable is reference frame dependent, its fluctuations
around the expectation value, or the whole statistics of results
from projective measurements of an observable for a fixed state,
would also be. The issue of a quantum frame of references is about
the latter. The quantum fluctuations of a measuring equipment,
for example, would give results with fluctuations even when
measuring what we believe to be a classical object, or what we
otherwise considered an eigenstate. We do not want to rush to the
conclusion that the notion of a system being quantum or classical
is relative. At least all of us humans seem to be classical enough to
one another, and most of the macroscopic world looks classical to
us too. Classical means, here with negligible quantum fluctuations
and entanglement. It suffices to say that quantum frames of
reference and transformations are worthy of serious studies.
